Recently, as a catalyst for purifying NOx in an exhaust gas of an oxygen-excessive lean atmosphere, an NOx sorption-and-reduction type catalyst has been utilized. This NOx sorption-and-reduction type catalyst, for example, as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,625, is one in which a noble metal, such as Pt and Rh, and an NOx sorption member, such as K and Ba, are loaded on a porous support, such as Al2O3. By using this NOx sorption-and-reduction type catalyst and controlling an air-fuel ratio from a lean side to a stoichiometric as well as rich side in a pulsating manner (rich spiking), since an exhaust gas, too, becomes from a lean atmosphere to a stoichiometric as well as rich atmosphere, NOx are sorbed in the NOx sorption member on the lean side, and they are released on the stoichiometric or rich side to react with reductive components, such as HC and CO, so that they are purified. Therefore, even when it is an exhaust gas from a lean burn engine, it is possible to purify NOx with good efficiency. Moreover, since HC and CO in the exhaust gas are oxidized by the noble metal and, at the same time, are consumed in the reduction of NOx, HC as well as CO are also purified with good efficiency.
For example, in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, a catalyst for purifying an exhaust gas is proposed in which an alkaline-earth metal, such as Ba, and Pt are loaded on a porous oxide support, such as Al2O3. Further, in Japanese Unexamined Patent Publication (KOKAI) No. 6-31,139, a catalyst for purifying an exhaust gas is proposed in which an alkali metal, such as K, and Pt are loaded on a porous oxide support, such as Al2O3. Furthermore, in Japanese Unexamined Patent Publication (KOKAI) No. 5-168,860, a catalyst for purifying an exhaust gas is proposed in which a rare-earth element, such as La, and Pt are loaded on a porous oxide support, such as Al2O3.
However, in an exhaust gas, SO2, which is generated by burning sulfur (S) contained in a fuel, is included, and it is oxidized by a noble metal in an exhaust gas of a lean atmosphere so that it is turned into SO3. Then, it has become apparent that it is readily turned into sulfuric acid by water vapor, which is also contained in the exhaust gas, that these react with the NOx sorption member to generate sulfites and sulfates, and that the NOx sorption member is thereby poisoned to degrade. This is referred to as the sulfur poisoning of the NOx sorption member. Moreover, since the porous oxide support, such as Al2O3, has a property that it is likely to adsorb SOx, there is a problem in that the aforementioned sulfur poisoning has been facilitated. Then, when the NOx sorption member is thus turned into sulfites and sulfates so that it is poisoned to degrade, it can no longer sorb NOx, as a result, there is a drawback in that the NOx purifying performance lowers after a high-temperature durability test (hereinafter, referred to as “post-durability”).
Moreover, it has been understood that another cause, by which the post-durability NOx purifying ability is lowered, lies in that a reaction occurs between cordierite, which has been used as a support substrate in general, and an alkali component, which works as the NOx sorption member, and thereby the NOx sorption ability has been lowered.
Namely, the alkali component is likely to move in a coating layer, such as Al2O3, and simultaneously exhibits a high reactivity to Si. Accordingly, it is believed that the alkali component moves in the coating layer when the temperature is high, arrives at the support substrate and reacts with Si in cordierite to form silicate, being a composite oxide. Since this silicate is a stable compound, the alkali component, which has been turned into silicate, no longer has an ability of sorbing NOx, and accordingly the NOx purifying performance has been lowered.
Moreover, it was thought of using a TiO2 support, and an experiment was carried out. As a result, since SOx flowed as they were to downstream without being adsorbed onto TiO2, and since only SO2, which contacted directly with a noble metal, was oxidized, it became apparent that the extent of sulfur poisoning was less. However, in a TiO2 support, the initial activity is low, moreover, since TiO2 reacts to an alkali component to generate titanates, it has become apparent that the post-durability NOx purifying performance is kept being low. Hence, in Japanese Unexamined Patent Publication (KOKAI) No. 8-099,034, there is proposed to use a composite support comprising TiO2—Al2O3. Moreover, it has been proposed to use a support in which rutile type TiO2 is added to Al2O3. However, even when a support made of composite oxide on made by thus adding TiO2 is used, it is difficult to suppress the reaction between an alkali component and TiO2.
While, in Japanese Unexamined Patent Publication (KOKAI) No. 8-117,602, an NOx sorption-and-reduction type catalyst is proposed in which a Ti—Zr composite oxide, an NOx sorption member and a noble metal are loaded on an Al2O3 support.
By thus making a composite support in which a Ti—Zr composite oxide is loaded on Al2O3, it is possible to highly maintain the initial NOx purifying rate due to the advantage of Al2O3. Moreover, by loading a Ti—Zr composite oxide, it is possible to raise the acidity of the support itself. Therefore, since this composite support, compared with the case where Al2O3 is used alone, is less likely to adsorb SOx, and since adsorbed SOx are likely to eliminate at a low temperature, the contacting probability between the NOx sorption member and SOx is lowered. In addition, in a support on which a Ti—Zr composite oxide is loaded, it is believed that the movement of alkali component is suppressed, and it has been understood that the reaction of the alkali component to TiO2 is suppressed. Therefore, when the aforementioned composite support is used, a high NOx purifying rate is secured even in the initial period, since the sulfur poisoning as well as the solving of the alkali metal into the support are suppressed, a post-durability NOx purifying rate is improved.
However, in a catalyst in which a Ti—Zr composite oxide is loaded on an Al2O3 support, compared with a composite support comprising TiO2—Al2O3 as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 8-099,034 or a catalyst which uses a support in which TiO2 is added to Al2O3, it is not possible to say that the suppression effect of the sulfur poisoning is sufficient, and accordingly it has been required to further suppress the sulfur poisoning and improve the NOx purifying rate.
Moreover, due to the recent improvement of engine performance, the increase of high speed driving, and the like, the temperature of exhaust gas has risen so that the improvement of NOx purifying activity in a high temperature region has become an assignment. This is because, in a high temperature region, NOx, which have been sorbed in an NOx sorption member, are likely to be released, and the NOx sorption amount is likely to be insufficient in a lean atmosphere. Hence, K, which exhibits a high basicity and is stable at a high temperature, is made into an NOx sorption member, but, in a catalyst in which K is loaded on Al2O3, and so on, there has been a problem in that the NOx sorption ability in a high temperature region is not improved so much.
Hence, it was thought of using a support which exhibits a higher basicity, and, in Japanese Unexamined Patent Publication (KOKAI) No. 5-195,755, a catalyst is disclosed in which K and a noble metal are loaded on ZrO2. In accordance with this catalyst, since the basicities of the support and NOx sorption member are high, it is likely to sorb but less likely to release NOx. Therefore, the NOx sorption ability in a high temperature region is improved, as a result, the NOx purifying performance in a high temperature region is improved.
However, in the catalyst in which K and a noble metal are loaded on ZrO2, since the basicity of the support is high, there has been a drawback in that it is likely to sorb not only NOx but also SOx so that the lowering of the activity by the sulfur poisoning is considerable. Namely, when the basicity of a support is high, although the NOx sorption ability is improved, even the sulfur poisoning is facilitated. On the contrary, when the basicity of a support is low, although the sulfur poisoning is suppressed, even the NOx sorption ability is lowered.
The present invention has been done in view of such circumstances, it is a main object to further suppress the sulfur poisoning of an NOx sorption-and-reduction type catalyst, thereby furthermore improving the high-temperature post-durability NOx purifying rate.
Moreover, the second object of the present invention is to make a catalyst which can satisfy both of the contradictory phenomena, such as the improvement of the NOx sorption ability in a high temperature region and the suppression of the sulfur poisoning, and which is good in terms of the heat resistance.
The characteristic of a catalyst for purifying an exhaust gas, of an NOx sorption-and-reduction type catalyst comprising a noble metal and an NOx sorption member loaded on a porous oxide support, which solves the aforementioned assignments and is set forth in claim 1, lies in that the porous oxide support includes composite particles having a structure comprising a core portion in which ZrO2 is a major component and a superficial portion formed on a surface of the core portion and including an oxide, whose basicity is lower than ZrO2, more than the core portion.
Moreover, the characteristic of a catalyst for purifying an exhaust gas, of an NOx sorption-and-reduction type catalyst comprising a support substrate, a coating layer formed on the support substrate and comprising a porous oxide, a noble metal loaded on the coating layer and at least one NOx sorption member loaded on the coating layer and selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements, which solves the aforementioned assignments and is set forth in claim 9, lies in that composite particles, having a structure comprising a core portion in which ZrO2 is a major component and a superficial portion formed on a surface of the core portion and including an oxide, whose basicity is lower than ZrO2, more than the core portion, are included in the coating layer.
In the present catalyst for purifying an exhaust gas, it is preferred that the oxide, whose basicity is lower than ZrO2, can be Al2O3, and that the composite particles can be a ZrO2—Al2O3 composite oxide. In this case, a composition of the composite particles can desirably fall in a range of Al2O3/ZrO2=3/97–15/85 by molar ratio.
Further, it is preferred that the oxide, whose basicity is lower than ZrO2, can be TiO2, and that the composite particles can be a ZrO2—TiO2 composite oxide. In this case, TiO2 can desirably be included in an amount of 30% by weight or more in the superficial portion of the composite particles. Furthermore, the composite particles can desirably further include Al2O3, and Al2O3 can desirably be included in an amount of 10% by mol or more in said composite particles.
Then, in the catalyst for purifying an exhaust gas of the present invention, an average particle diameter of the composite particles can desirably be 10 μm or less.